US-backed Kurdish forces have pleaded with Washington to halt a Turkish offensive against them in Syria as it prepared to send reinforcements to the region.

Turkey's forces have crossed into northern Syria to assault the Kurdish-held area of Afrin, raising alarm in Western capitals that Turkish troops are attacking the same fighters who helped defeat Isis (Islamic State). Activists say Turkey has also mobilised 10,000 Syrian fighters to storm Afrin.

Turkey says it is targeting the Kurdish People's Protection Units (YPG), which it considers to be a terrorist group. But YPG fighters also make up the bulk of the Syrian Democratic Forces (SDF), the United States-backed rebel group who drove Isis out of its capital in Raqqa.

As Turkish forces intensified their attack, the SDF pleaded with their US allies to restrain Turkey. "The coalition is urged to take its responsibilities towards our forces and our people in Afrin," said Keno Gabriel, an SDF spokesman. He warned that the Turkish attack on Kurdish troops would only benefit the remaining fragments of Isis and give them space to regroup.

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The US has offered direct military and logistical support to the SDF and has said it will create a 30,000-strong border force of existing Kurdish and Arab SDF members to ensure there would be no Isis comeback.

Turkey, meanwhile, has urged the US to stop backing the YPG in Syria.

Thousands of civilians are reportedly trying to flee Afrin, and Syrian activists say more than 70 people have died since the Turkish push began on Saturday.